Hdpe

ABSTRACT

A multimodal polyethylene copolymer comprising:
         (III) 35 to 55 wt % of a lower molecular weight ethylene homopolymer component having an MFR 2  of 200 to 400 g/10 min;   (IV) 45 to 65 wt % of a higher molecular weight ethylene copolymer component of ethylene and at least one C3-12 alpha olefin comonomer;   wherein said multimodal polyethylene copolymer has a density of 955 to 962 kg/m 3 , an MFR 2  of 2.0 to 10 g/10 min and an Mw/Mn of more than 9.0.

The present invention relates to a high density polyethylene polymer forcompression or injection moulded articles, in particular for themanufacture of caps and closures. The present invention also relates toa process for the production of said polymer, a process for theproduction of a compression or injection moulded article comprising saidpolymer and to the use of said polymer for the production of acompression or injection moulded article such as a cap or closure.

The polyethylene of the invention is a multimodal high densitypolyethylene copolymer with a desirable balance of rheological,processing and mechanical properties. The specific combination ofpolymer design parameters (e.g. the molecular weights of lower andhigher molecular weight fractions and distribution thereof, as well asthe split between the fractions and final MFR and density) were found toprovide a polymer with good flow, good rheological and good mechanicalproperties, e.g. in terms of environmental stress crack resistance(ESCR), impact strength and stiffness.

BACKGROUND

Injection moulding may be used to make a wide variety of articlesincluding articles having relatively complex shapes and a range ofsizes. Injection moulding is, for instance, suited to the manufacture ofarticles used as caps and closures for food and drink applications, suchas for bottles containing carbonated or non-carbonated drinks, or fornon-food applications like containers for cosmetics and pharmaceuticals.

Injection moulding is a moulding process in which a polymer is meltedand then filled into a mould by injection. During initial injection,high pressure is used and the polymer melt is compressed. Thus, uponinjection into the mould the polymer melt initially expands or “relaxes”to fill the mould. The mould, however, is kept at a lower temperaturethan the polymer melt. As the polymer melt cools, shrinkage tends tooccur. To compensate for this effect, back pressure is applied.Thereafter the polymer melt is cooled further to enable the mouldedarticle to be removed from the mould without causing deformation.

An important property of an injection moulded article is its stresscrack resistance. It will be appreciated that the injection mouldedarticles of the invention should not exhibit brittle failure and shouldtherefore possess a high stress crack resistance. The present inventorstherefore sought new HDPEs, developed in particular for the cap andclosure market, which possess improved stress cracking resistance.

The HDPE should also offer injection moulded articles with high impactstrength so the articles withstand being dropped and withstandtransportation.

It is also important that produced articles (e.g. caps or closures)having sufficient mechanical strength (rigidity) implying that the HDPEresin should have sufficient stiffness.

Improvements in these mechanical properties must not be at the expenseof processability of the polymer. Processability must be maintained oreven improved to meet customer needs. Injection moulded articles areproduced rapidly and any reduction in processability can increase cycletimes and hence reduce process efficiency. To achieve high productionspeed and hence a process with improved economics, it is important forthe resin to have very good flow, in particular flow under pressure(i.e. good rheology).

It is therefore required that the HDPE resin which is used to producebeverage cap or closure has a balance of mechanical and rheologicalproperties. The improvement of one of the properties above however,leads to a reduction in another equally valuable property. For example,stiffness can be increased by increasing polymer density. Unfortunatelythis will reduce the resistance to crack propagation. Likewise, resinflow can be improved by increasing its melt flow rate but only at theexpense of impact resistance. Therefore, it is not easy to achieve thedesired balance required properties.

The invention also relates to the preparation of compression mouldedarticles. Compression moulding is a method of moulding in which themolding material, generally preheated, is first placed in an open,heated mould cavity. The mould is closed, pressure is applied to forcethe material into contact with all mould areas, while heat and pressureare maintained until the moulding material has set. In injection moldingprocesses, the polymer melt flows under high shear rates and flowability measured by spiral flow provides a good indication on how thepolymer will flow in the mold. In compression molding processes thesituation is different. As the polymer melt is not subjected to highshear rates, polymer flow is proportional to MFR of the resin. It ispossible that a low MFR HDPE has high spiral flow (due to high shearthinning) but this will not necessarily translate to good flow in acompression molding process. In this respect, the high melt flow of theinventive examples is beneficial to achieve good flow in a compressionmolding process while high spiral flow gives good flow ability in aninjection molding process. Many prior art examples fail to achieve suchcombination.

The present inventors have therefore devised a narrowly definedmultimodal HDPE copolymer that possesses excellent mechanicalproperties, good flow for both compression and injection moulding andhigh processability.

Some bimodal HDPE compositions are known. In EP-A-1,565,525 theinventors describe a bimodal HDPE, preferably for blow mouldingapplications. It has a high molecular weight component which possesseshigh short chain branching. That is achieved however using a single sitecatalyst and results in a narrow Mw/Mn. The narrow Mw/Mn makes thesepolymers less processable and they have poor flow.

WO2010/022941 describes an HDPE for injection moulding possessing goodESCR and flow. The polymer is however, made using an obscure dualsitecatalyst and relies on a catalyst comprising a single site catalyst andan iron based catalyst. Such a catalyst is not favoured industrially, asa process involving a dualsite catalyst is necessarily limiting as theconditions cannot be varied between polymerisation stages. The formedpolymer in '941 is one based on two copolymer fractions and it isimpossible to prepare a homopolymer/copolymer using a dualsite catalystas described in '941.

WO2013/045663 describes bimodal HDPE's with low melt index with goodrigidity and ESCR. These can be used for caps and closures. Unusually,the polymer is made with the lower density component in the first stageof the process, that component having low melt index. In the examples,no experimental conditions or comonomers are mentioned, although thefinal blend has a low MFR₂.

WO2014/180989 describes multimodal polymers for cap and closuremanufacture with excellent stress crack and tensile properties that leadto a reduction of angel hair and high tips on forming caps. Thesepolymers have low MFR.

JP4942525 describes a polyethylene resin composition for a bottle caphaving an excellent rigidity and excellent high speed molding propertywithout reducing environmental stress cracking resistance. The polymeris a multimodal HDPE having homopolymer and copolymer components and anMFR of 5.0 to 10.0 g/10 min, a density is 0.960 to 0.967 g/cm³, and anMw/Mn of 8.0 to 12.0.

WO03/039984 describes screw caps made from a bimodal polyethylene inwhich an ethylene homopolymer is combined with an ethylene copolymerfraction. The caps possess good ESCR, injectability and impactresistance. The polyethylenes therein are generally of lower MFR₂ thanwe require and are primarily targeted at caps for carbonated beverages.Such caps require very high ESCR which is achieved by keeping melt flowand densities at lower level.

The present inventors have found that a particular combination ofproperties leads to an ideal balance of mechanical properties,rheological properties and processability. By manufacturing a multimodalHDPE with high melt index having an ethylene homopolymer lower molecularweight fraction in combination with a higher molecular weight copolymerfraction gives excellent properties. The invention has been compared toa broad range of commercial moulding HDPE grades of comparable densitiesto show that the relationships in claim 1 are not ones which can befound in commercial polymers and which yield the advantageous propertieshighlighted above.

The combination of tailored flow, good stiffness, good rheology and goodstress crack is achieved using a blend in which the LMW ethylenehomopolymer has very high melt index (and hence low Mw) and high densitycombined with a HMW component that results in an overall density andMFR₂ which is higher than some conventional solutions. Surprising,despite the higher density we maintain high ESCR which might be expectedto fall. Our flow is also in the ideal range for injection moulding. Theinvention is ideally suited to the manufacture of caps fornon-carbonated beverage containers as the inventive compositions providevery good balance between flow ability, stiffness and stress crackingresistance.

SUMMARY OF INVENTION

Viewed from one aspect the invention provides a multimodal polyethylenecopolymer comprising:

-   -   (I) 35 to 55 wt % of a lower molecular weight ethylene        homopolymer component having an MFR₂ of 200 to 400 g/10 min;    -   (II) 45 to 65 wt % of a higher molecular weight ethylene        copolymer component of ethylene and at least one C3-12 alpha        olefin comonomer;

wherein said multimodal polyethylene copolymer has a density of 955 to962 kg/m³, an MFR, of 2.0 to 10.0 g/10 min and an Mw/Mn of more than9.0.

The multimodal polyethylene copolymer of the invention is made using aZiegler Natta catalyst.

Viewed from another aspect the invention provides an injection orcompression moulded article, such as a cap or closure, comprising amultimodal polyethylene copolymer as herein before defined. Such caps orclosures may weight from 1 to 10 g.

Viewed from another aspect the invention provides the use of themultimodal polyethylene copolymer as hereinbefore defined in themanufacture of an injection moulded or compression article, such as acap or closure.

Viewed from another aspect the invention provides a process for thepreparation of a multimodal polyethylene copolymer as herein beforedefined comprising;

-   -   polymerising ethylene in the presence of a Ziegler Natta        catalyst so as to form said lower molecular weight homopolymer        component (I); and subsequently

polymerising ethylene and at least one C3-12 alpha olefin comonomer inthe presence of component (I) and in the presence of the same ZieglerNatta catalyst so as to form said higher molecular weight component (II)and hence form said multimodal polyethylene copolymer as herein beforedefined.

The invention further comprises compression or injection moulding theproduct of said process to form an article, such as a cap or closure.

Definitions

The multimodal polyethylene copolymer of the invention comprises aethylene homopolymer component or fraction which contains only ethylenemonomer residues and a polyethylene copolymer component or fractionwhich comprises ethylene copolymerised with at least one C3-12 alphaolefin. The terms component and fraction can be used interchangeablyherein.

All parameters mentioned above and below are measured according to testmethods set out before the examples.

DETAILED DESCRIPTION OF INVENTION

It has been found that the high density polyethylene copolymer accordingto the invention provides an improved material for compression orespecially injection moulding, in particular for cap and closureapplications, which combines very good mechanical properties e.g. interms of ESCR, with excellent processability and ideal flow, e.g. interms of shear thinning index and spiral flow.

The polymer of the invention is a multimodal high density ethylenecopolymer as it contains an ethylene copolymer fraction. In an ethylenecopolymer fraction, the majority by mole of monomer residues present arederived from ethylene monomer units. The comonomer contribution in theHMW component preferably is up to 10% by mol, more preferably up to 5%by mol in any copolymer fraction. Ideally, however there are very lowlevels of comonomer present in any copolymer fraction such as 0.1 to 3.0mol %, e.g. 0.5 to 2.0 mol %.

The overall comonomer content in the multimodal polyethylene copolymeras a whole may be 0.05 to 3.0 mol % e.g. 0.1 to 2.0 mol %, preferably0.2 to 1.0 mol %.

The copolymerisable monomer or monomers present in any copolymercomponent are C3-12 alpha olefin comonomers, particularly singly ormultiply ethylenically unsaturated comonomers, in particular C3-12-alphaolefins such as propene, but-1-ene, hex-1-ene, oct-1-ene, and4-methyl-pent-1-ene. The use of 1-hexene and 1-butene is particularlypreferred. Ideally there is only one comonomer present. Ideally thatcomonomer is 1-butene.

The polymer of the invention is multimodal and therefore comprises atleast two components. It is generally preferred if the higher molecularweight (HMW) component has an Mw of at least 5000 more than the lowermolecular weight (LMW) component, such as at least 10,000 more.Alternatively viewed, the MFR₂ of the HMW component is lower than theMFR₂ of the LMW component.

The HDPE of the invention is multimodal. Usually, a polyethylenecomposition comprising at least two polyethylene fractions, which havebeen produced under different polymerisation conditions resulting indifferent (weight average) molecular weights and molecular weightdistributions for the fractions, is referred to as “multimodal”.Accordingly, in this sense the compositions of the invention aremultimodal polyethylenes. The prefix “multi” relates to the number ofdifferent polymer fractions the composition is consisting of. Thus, forexample, a composition consisting of two fractions only is called“bimodal”.

The form of the molecular weight distribution curve, i.e. the appearanceof the graph of the polymer weight fraction as function of its molecularweight, of such a multimodal polyethylene will show two or more maximaor at least be distinctly broadened in comparison with the curves forthe individual fractions.

For example, if a polymer is produced in a sequential multistageprocess, utilising reactors coupled in series and using differentconditions in each reactor, the polymer fractions produced in thedifferent reactors will each have their own molecular weightdistribution and weight average molecular weight. When the molecularweight distribution curve of such a polymer is recorded, the individualcurves from these fractions are superimposed into the molecular weightdistribution curve for the total resulting polymer product, usuallyyielding a curve with two or more distinct maxima.

It is preferred if the polymer of the invention is bimodal.

The polymer of the invention has an MFR₂ of 2.0 to 10.0 g/10 min,preferably 2.0 to 5.0 g/10 min. The polymer preferably has an MFR₂ of2.0 to 4.9 g/10 min. Most preferably, the MFR₂ may be of 2.5 to 4.9 g/10min, preferably 3.0 to 4.9 g/10 min.

The polymer of the invention preferably has an MFR_(S) of 11.0 to 18.0g/10 min., preferably 12-16 g/10 min.

The density of the multimodal ethylene copolymer is 955 to 962 kg/m³.The polymers of the invention are therefore high density polyethylenes,HDPE. More preferably, the polymer has a density of 956 to 962 kg/m³,such as 956 to 960 kg/m³, especially 956 to 959 kg/m³. In certainembodiments the density of the copolymer of the invention is less than960 kg/m³.

Preferably, the polymers of the invention possess an ESCR of 20 hrs ormore, such as 25 hrs or more. An ESCR in the range of 20 to 70 hrs,especially 25 to 59 hrs is preferred.

It will be appreciated that the molecular weight and moleculardistribution of the polymers of the invention is important. Thepolyethylene polymer has a molecular weight distribution Mw/Mn, beingthe ratio of the weight average molecular weight Mw and the numberaverage molecular weight Mn, of more than 9.0, more preferably more than10.0, such as 10.0 to 20.0, preferably 10.5 to 18.0.

The multimodal ethylene copolymer preferably has an Mw/Mn of 30.0 orbelow, more preferably of 25.0 or below, even more preferably of 20.0 orbelow.

The weight average molecular weight Mw of the multimodal ethylenecopolymer of the invention preferably is at least 50,000, morepreferably at least 70,000. Furthermore, the Mw of the compositionpreferably is at most 200,000, more preferably at most 150,000.

The Mz of the multimodal polyethylene copolymer of the invention ispreferably 300,000 to 500,000. The ratio of Mz/Mw is preferably in therange of 2.0 to 7.0, such as 3.5 to 5.5.

The shear thinning index (SHI 2.7/210) may be at least 6.0, such as atleast 7.0. Ideally the SHI is in the range of 7.0 to 18.0, preferably8.0 to 15.0.

The charpy impact strength measured at 23° C. may be 4.0 to 10.0 kJ/m²,such as 5.0 to 8.0 kJ/m².

The flow properties of the multimodal polyethylene copolymer of theinvention are important. Too little flow or too much flow limits theutility of the polyethylene. At 190° C., the spiral flow length at 600bar pressure may be 9.0 to 15.0 cm, preferably 10.0 to 14.0 cm. Underthe same conditions of 190° C. at 1000 bar pressure, the length may be14.0 to 21.0 cm, preferably 15.0 to 19.0 cm, especially 16.0 to 19.0 cm.At 1400 bar pressure, 190° C., flow length may be 18.0 to 25.0 cm,preferably 19.0 to 24.0 cm.

At 220° C., flow length at 600 bar may be 10.0 to 15.0 cm, preferably11.0 to 14.0 cm. At 1000 bars flow length may be 16.0 to 24.0 cm,preferably 17.0 to 22.0 cm, especially 17.5 to 20.0 cm. At 1400 barsflow length may be 20.0 to 30.0 cm, preferably 22.0 to 26.0 cm.

It is the combination of high density, high overall MFR₂, high MFR₂ inthe LMW component and broad Mw/Mn that contributes to the excellent flowproperties, good rheology and good ESCR that we observe. The high finalMFR and high loop MFR are important to achieve good rheologicalproperties and consequently good flowability both for compression andinjection moulding. High density of LMW and final bimodal copolymer,bimodality (Mw/Mn) and homopolymer/copolymer component design (includingthe split) is important to achieve good stiffness-ESCR combination.

As noted above, the polymers of the invention preferably comprise alower molecular weight component (I) and a higher molecular weightcomponent (II). The weight ratio of LMW fraction (I) to HMW fraction(II) in the composition is in the range 35:65 to 55:45, more preferably40:60 to 55:45, most preferably 48:52 to 52:48. It has been foundtherefore that the best results are obtained when the HMW component ispresent at around the same percentage as the LMW component or evenpredominates, e.g. 48 to 52 wt % of the HMW component (II) and 52 to 48wt % fraction (I).

An ideal polymer is therefore a lower molecular weight homopolymercomponent (I) with a higher molecular weight component (II) which is anethylene 1-butene component.

The lower molecular weight fraction (I) has an MFR₂ of 200 to 400 g/10min g/10 min. A range of 250 to 350 g/10 min is preferred. This highMFR₂ in the LMW fraction ensures that there is a large difference in Mwbetween LMW and HMW components and is important in giving the multimodalpolyethylene copolymer of the invention the good rheological propertiesand ideal flow as well as good ESCR which we observe.

Fraction (I) is an ethylene homopolymer with a preferred density of 965to 975 kg/m³, preferably 968 to 972 kg/m³.

The HMW component is an ethylene copolymer. Its properties are chosensuch that the desired final density and MFR are achieved. It has a lowerMFR₂ than the LMW component and a lower density. Ideally it is acopolymer of ethylene and 1-butene.

Where herein features of fractions (I) and/or (II) of the composition ofthe present invention are given, these values are generally valid forthe cases in which they can be directly measured on the respectivefraction, e.g. when the fraction is separately produced or produced inthe first stage of a multistage process. However, the composition mayalso be and preferably is produced in a multistage process wherein e.g.fractions (I) and (II) are produced in subsequent stages. In such acase, the properties of the fractions produced in the second step (orfurther steps) of the multistage process can either be inferred frompolymers, which are separately produced in a single stage by applyingidentical polymerisation conditions (e.g. identical temperature, partialpressures of the reactants/diluents, suspension medium, reaction time)with regard to the stage of the multistage process in which the fractionis produced, and by using a catalyst on which no previously producedpolymer is present. Alternatively, the properties of the fractionsproduced in a higher stage of the multistage process may also becalculated based on the properties of both the fraction produced inprior stage and the final product, e.g. in accordance with B. Hagström,Conference on Polymer Processing (The Polymer Processing Society),Extended Abstracts and Final Programme, Gothenburg, Aug. 19 to 21, 1997,4:13.

Thus, although not directly measurable on the multistage processproducts, the properties of the fractions produced in higher stages ofsuch a multistage process can be determined by applying either or bothof the above methods. The skilled person will be able to select theappropriate method.

A multimodal (e.g. bimodal) polyethylene as hereinbefore described maybe produced by mechanical blending two or more polyethylenes (e.g.monomodal polyethylenes) having differently centred maxima in theirmolecular weight distributions. The monomodal polyethylenes required forblending may be available commercially or may be prepared using anyconventional procedure known to the skilled man in the art. Each of thepolyethylenes used in a blend and/or the final polymer composition mayhave the properties hereinbefore described for the lower molecularweight component, and higher molecular weight component of thecomposition, respectively.

However, it is preferred if the copolymer of the invention is formed ina multistage process. The process of the invention preferably involves

polymerising ethylene so as to form a lower molecular weight homopolymercomponent (I) as herein defined; and subsequently

polymerising ethylene and at least one C3-12 alpha olefin comonomer inthe presence of component (I) so as to form a higher molecular weightcomponent (II) and hence to form the desired multimodal polyethylenecopolymer of the invention. The same Ziegler Natta catalyst is used inboth stages of the process and is transferred from step (I) to step (II)along with component (I).

It is preferred if at least one component is produced in a gas-phasereaction.

Further preferred, one of the fractions (I) and (II) of the polyethylenecomposition, preferably fraction (I), is produced in a slurry reaction,preferably in a loop reactor, and one of the fractions (I) and (II),preferably fraction (II), is produced in a gas-phase reaction.

Preferably, the multimodal polyethylene composition may be produced bypolymerisation using conditions which create a multimodal (e.g. bimodal)polymer product using a Ziegler Natta catalyst system using a two ormore stage, i.e. multistage, polymerisation process with differentprocess conditions in the different stages or zones (e.g. differenttemperatures, pressures, polymerisation media, hydrogen partialpressures, etc).

Preferably, the multimodal (e.g. bimodal) composition is produced by amultistage ethylene polymerisation, e.g. using a series of reactors,with optional comonomer addition preferably in only the reactor(s) usedfor production of the higher/highest molecular weight component(s). Amultistage process is defined to be a polymerisation process in which apolymer comprising two or more fractions is produced by producing eachor at least two polymer fraction(s) in a separate reaction stage,usually with different reaction conditions in each stage, in thepresence of the reaction product of the previous stage which comprises apolymerisation catalyst. The polymerisation reactions used in each stagemay involve conventional ethylene homopolymerisation or copolymerisationreactions, e.g. gas-phase, slurry phase, liquid phase polymerisations,using conventional reactors, e.g. loop reactors, gas phase reactors,batch reactors etc. (see for example WO97/44371 and WO96/18662).

Polymer compositions produced in a multistage process are alsodesignated as “in-situ” blends.

It is previously known to produce multimodal, in particular bimodal,olefin polymers, such as multimodal polyethylene, in a multistageprocess comprising two or more reactors connected in series. As instanceof this prior art, mention may be made of EP 517 868, which is herebyincorporated by way of reference in its entirety, including all itspreferred embodiments as described therein, as a preferred multistageprocess for the production of the polyethylene composition according tothe invention.

Preferably, the main polymerisation stages of the multistage process forproducing the composition according to the invention are such asdescribed in EP 517 868, i.e. the production of fractions (I) and (II)is carried out as a combination of slurry polymerisation for fraction(0/gas-phase polymerisation for fraction (II). The slurry polymerisationis preferably performed in a so-called loop reactor. Further preferred,the slurry polymerisation stage precedes the gas phase stage.

Optionally and advantageously, the main polymerisation stages may bepreceded by a prepolymerisation, in which case up to 20% by weight,preferably 1 to 10% by weight, more preferably 1 to 5% by weight, of thetotal composition is produced. The prepolymer is preferably an ethylenehomopolymer (High Density PE). At the prepolymerisation, preferably allof the catalyst is charged into a loop reactor and the prepolymerisationis performed as a slurry polymerisation. Such a prepolymerisation leadsto less fine particles being produced in the following reactors and to amore homogeneous product being obtained in the end. Any prepolymer isconsidered a part of the LMW component herein.

The polymerisation catalyst is a Ziegler-Natta (ZN) catalyst. Thecatalyst may be supported, e.g. with conventional supports includingmagnesium dichloride based supports or silica. Preferably the catalystis a ZN catalyst, more preferably the catalyst is silica supported ZNcatalyst.

The Ziegler-Natta catalyst further preferably comprises a group 4 (groupnumbering according to new IUPAC system) metal compound, preferablytitanium, magnesium dichloride and aluminium.

The catalyst may be commercially available or be produced in accordanceor analogously to the literature. For the preparation of the preferablecatalyst usable in the invention reference is made to WO2004055068 andWO2004055069 of Borealis, EP 0 688 794 and EP 0 810 235. The content ofthese documents in its entirety is incorporated herein by reference, inparticular concerning the general and all preferred embodiments of thecatalysts described therein as well as the methods for the production ofthe catalysts. Particularly preferred Ziegler-Natta catalysts aredescribed in EP 0 810 235.

The resulting end product consists of an intimate mixture of thepolymers from the two or more reactors, the differentmolecular-weight-distribution curves of these polymers together forminga molecular-weight-distribution curve having a broad maximum or two ormore maxima, i.e. the end product is a multimodal polymer mixture, suchas bimodal mixture.

It is preferred that the base resin, i.e. the entirety of all polymericconstituents, of the composition according to the invention is a bimodalpolyethylene mixture consisting of fractions (I) and (II), optionallyfurther comprising a small prepolymerisation fraction in the amount asdescribed above. It is also preferred that this bimodal polymer mixturehas been produced by polymerisation as described above under differentpolymerisation conditions in two or more polymerisation reactorsconnected in series. Owing to the flexibility with respect to reactionconditions thus obtained, it is most preferred that the polymerisationis carried out in a loop reactor/a gas-phase reactor combination.

Preferably, the polymerisation conditions in the preferred two-stagemethod are so chosen that the comparatively lower molecular polymerhaving no content of comonomer is produced in one stage, preferably thefirst stage, owing to a high content of chain-transfer agent (hydrogengas), whereas the higher molecular polymer having a content of comonomeris produced in another stage, preferably the second stage.

In the preferred embodiment of the polymerisation in a loop reactorfollowed by a gas-phase reactor, the polymerisation temperature in theloop reactor preferably is 85 to 115° C., more preferably is 90 to 105°C., and most preferably is 92 to 100° C., and the temperature in thegas-phase reactor preferably is 70 to 105° C., more preferably is 75 to100° C., and most preferably is 82 to 97° C.

A chain-transfer agent, preferably hydrogen, is added as required to thereactors, and preferably 100 to 800 moles of H₂/kmoles of ethylene areadded to the reactor, when the LMW fraction is produced in this reactor,and 50 to 500 moles of H₂/kmoles of ethylene are added to the gas phasereactor when this reactor is producing the HMW fraction.

In the production of the composition of the present invention,preferably a compounding step is applied, wherein the composition of thebase resin, i.e. the blend, which is typically obtained as a base resinpowder from the reactor, is extruded in an extruder and then pelletisedto polymer pellets in a manner known in the art.

The polyethylene composition, e.g. in pellet form, may also containminor quantities of additives such as pigments, nucleating agents,antistatic agents, fillers, antioxidants, etc., generally in amounts ofup to 10% by weight, preferably up to 5% by weight.

Optionally, additives or other polymer components can be added to thecomposition during the compounding step in the amount as describedabove. Preferably, the composition of the invention obtained from thereactor is compounded in the extruder together with additives in amanner known in the art.

The polyethylene polymer of the invention may also be combined withother polymer components such as other polymers of the invention, withother HDPEs or with other polymers such as LLDPE or LDPE. Howeverarticles of the invention such as caps and closures are preferably atleast 90 wt % of the polymer of the invention, such as at least 95 wt %.In one embodiment, the articles consist essentially of the polymer ofthe invention. The term consists essentially of means that the polymerof the invention is the only “non additive” polyolefin present. It willbe appreciated however that an article and also the polymer used to makeit may contain standard polymer additives some of which might besupported on a polyolefin (a so called masterbatch as is well known inthe art). The term consists essentially of does not exclude the presenceof such a supported additive.

Applications

Still further, the present invention relates to an injection orcompression moulded article, preferably a cap or closure, comprising apolyethylene composition as described above and to the use of such apolyethylene composition for the production of an injection orcompression moulded article, preferably a cap or closure. Preferably,injection moulded articles are made. The invention is ideally suited tothe manufacture of caps for containers such as still water. The caps ofthe invention is therefore ideal for bottles containing non-carbonateddrinks as the inventive compositions provide very good balance betweenflow ability (spiral flow), stiffness and stress cracking resistance forthat application.

Injection moulding of the composition hereinbefore described may becarried out using any conventional injection moulding equipment. Atypical injection moulding process may be carried out a temperature of190 to 275° C.

Still further, the present invention relates to a compression mouldedarticle, preferably a caps or closure article, comprising a polyethylenepolymer as described above and to the use of such a polyethylene polymerfor the production of a compression moulded article, preferably a cap orclosure.

Preferably, the composition of the invention is used for the productionof a caps or closure article.

As noted above, the caps and closures of the present invention areadvantageous not only because of their high ESCR properties, but alsobecause they have ideal flow.

The caps and closures of the invention are of conventional size,designed therefore for bottles and the like. They are approximately 2 to8 cm in outer diameter (measured across the solid top of the cap)depending on the bottle and provided with a screw. Cap height might be0.8 to 3 cm.

Caps and closure may be provided with tear strips from which the capdetaches on first opening as is well known in the art. Caps may also beprovided with liners.

It will be appreciated that any parameter mentioned above is measuredaccording to the detailed test given below. In any parameter where anarrower and broader embodiment are disclosed, those embodiments aredisclosed in connection with the narrower and broader embodiments ofother parameters.

The invention will now be described with reference to the following nonlimiting examples.

Test Methods: Melt Flow Rate

The melt flow rate (MFR) is determined according to ISO 1133 and isindicated in g/10 min. The MFR is an indication of the melt viscosity ofthe polymer. The MFR is determined at 190° C. The load under which themelt flow rate is determined is usually indicated as a subscript, forinstance MFR₂ is measured under 2.16 kg load (condition D), MFR₅ ismeasured under 5 kg load (condition T) or MFR₂₁ is measured under 21.6kg load (condition G).The quantity FRR (flow rate ratio) is an indication of molecular weightdistribution and denotes the ratio of flow rates at different loads.Thus, FRR_(21/2) denotes the value of MFR₂₁/MFR₂.

Density

Density of the polymer was measured according to ISO 1183/1872-2B.For the purpose of this invention the density of the blend can becalculated from the densities of the components according to:

$\rho_{b} = {\sum\limits_{i}{w_{i} \cdot \rho_{i}}}$

where ρ_(b) is the density of the blend,

-   -   w_(i) is the weight fraction of component “i” in the blend and    -   ρ_(i) is the density of the component “i”.

Molecular Weight

Molecular weight averages, molecular weight distribution (Mn, Mw, Mz,MWD)Molecular weight averages (Mz, Mw and Mn), Molecular weight distribution(MWD) and its broadness, described by Mw/Mn (wherein Mn is the numberaverage molecular weight and Mw is the weight average molecular weight)were determined by Gel Permeation Chromatography (GPC) according to ISO16014-1:2003, ISO 16014-2:2003, ISO 16014-4:2003 and ASTM D 6474-12using the following formulas:

$\begin{matrix}{M_{n} = \frac{\sum\limits_{i = 1}^{N}A_{i}}{\sum\limits_{i = 1}^{N}\left( {A_{i}/M_{i}} \right)}} & (1) \\{M_{w} = \frac{\sum\limits_{i = 1}^{N}\left( {A_{i} \times M_{i}} \right)}{\sum\limits_{i = 1}^{N}A_{i}}} & (2) \\{M_{z} = \frac{\sum\limits_{i = 1}^{N}\left( {A_{i} \times M_{i}^{2}} \right)}{\sum\limits_{i = 1}^{N}\left( {A_{i}/M_{i}} \right)}} & (3)\end{matrix}$

For a constant elution volume interval ΔV_(i), where A_(i), and M_(i)are the chromatographic peak slice area and polyolefin molecular weight(MW), respectively associated with the elution volume, V_(i), where N isequal to the number of data points obtained from the chromatogrambetween the integration limits.A high temperature GPC instrument, equipped with either infrared (IR)detector (IR4 or IR5 from PolymerChar (Valencia, Spain) or differentialrefractometer (RI) from Agilent Technologies, equipped with 3×Agilent-PLgel Olexis and 1× Agilent-PLgel Olexis Guard columns was used.As the solvent and mobile phase 1,2,4-trichlorobenzene (TCB) stabilizedwith 250 mg/L 2,6-Di tert butyl-4-methyl-phenol) was used. Thechromatographic system was operated at 160° C. and at a constant flowrate of 1 mL/min. 200 μL of sample solution was injected per analysis.Data collection was performed using either Agilent Cirrus softwareversion 3.3 or PolymerChar GPC-IR control software.The column set was calibrated using universal calibration (according toISO 16014-2:2003) with 19 narrow MWD polystyrene (PS) standards in therange of 0.5 kg/mol to 11 500 kg/mol. The PS standards were dissolved atroom temperature over several hours. The conversion of the polystyrenepeak molecular weight to polyolefin molecular weights is accomplished byusing the Mark Houwink equation and the following Mark Houwinkconstants:

K _(PS)=19×10⁻³ mL/g, α_(PS)=0.655

K _(PE)=39×10⁻³ mL/g, α_(PE)=0.725

K _(PP)=19×10⁻³ mL/g, α_(PP)=0.725

A third order polynomial fit was used to fit the calibration data.

All samples were prepared in the concentration range of 0.5-1 mg/ml anddissolved at 160° C. for 2.5 hours for PP or 3 hours for PE undercontinuous gentle shaking.

Spiral Flow: measured by using an Arburg Injection moulding machine:Arburg 320 C 600-250 tag 87-LT-E03,Axxicon base mould with spiral mould insert with a thickness of 1 mm and5 mm broadness,Processing parameters:

-   -   Melt temperature should be 190° C. and 220° C.    -   Injection cycle: injection time including holding: 15.5 sec    -   Cooling time: 15 sec    -   Holding pressure: 600 bar, 1000 bar and 1400 bar    -   Screw speed: 33 rpm    -   Injection speed: 50 mm/sec    -   Back pressure 7 bar (system pressure)    -   Melt cushion: 9 mm    -   Tool temperature: 40° C.    -   Plasticizing point: 41 mm    -   Switch over point: 28 mm        The spiral flow test was carried out in the following way:        Properties adjusted as written above.        The cylinder was purged very well before every material.        The test started every time and for every material with the        highest holding pressure.        Measurement of spiral flow was not done before the flow length        was stable.        Waited at least 10 shots before reading the flow length even it        could be stabile before.        Measured 5 injected spirals and calculated the average.

Environmental Stress Crack Resistance

Environmental stress crack resistance (ESCR) was determined according toASTM 1693, condition B at 50° C. and using 10% Igepal co-630.

Notched Charpy Impact Strength

Notched Charpy impact strength was measured according to ISO 179/1 eA at23° C. by using injection moulded test specimen (80×10×4 mm), moulded asdescribed in EN ISO 1873-2

Quantification of Microstructure by NMR Spectroscopy

Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used toquantify the comonomer content of the polymers.

Quantitative ¹³C{¹H} NMR spectra recorded in the molten-state using aBruker Advance III 500 NMR spectrometer operating at 500.13 and 125.76MHz for ¹H and ¹³C respectively. All spectra were recorded using a ¹³Coptimised 7 mm magic-angle spinning (MAS) probehead at 150° C. usingnitrogen gas for all pneumatics. Approximately 200 mg of material waspacked into a 7 mm outer diameter zirconia MAS rotor and spun at 4 kHz.Standard single-pulse excitation was employed utilising the transientNOE at short recycle delays of 3 s {pollard04, klimke06} and the RS-HEPTdecoupling scheme {fillip05, griffin07}. A total of 1024 (1 k)transients were acquired per spectrum. This setup was chosen due itshigh sensitivity towards low comonomer contents.

Quantitative ¹³C{¹H} NMR spectra were processed, integrated andquantitative properties determined using custom spectral analysisautomation programs. All chemical shifts are internally referenced tothe bulk methylene signal (δ+) at 30.00 ppm {randall89}.

Characteristic signals corresponding to the incorporation of 1-butenewere observed (randall89) and all contents calculated with respect toall other monomers present in the polymer.

Characteristic signals resulting from isolated 1-butene incorporationi.e. EEBEE comonomer sequences, were observed. Isolated 1-buteneincorporation was quantified using the integral of the signal at 39.84ppm assigned to the *B2 sites, accounting for the number of reportingsites per comonomer:

B=I _(*B2)

With no other signals indicative of other comonomer sequences, i.e.consecutive comonomer incorporation, observed the total 1-butenecomonomer content was calculated based solely on the amount of isolated1-butene sequences:

B _(total) =B

The relative content of ethylene was quantified using the integral ofthe bulk methylene (δ+) signals at 30.00 ppm:

E=(½)*I _(δ+)

The total ethylene comonomer content was calculated based the bulkmethylene signals and accounting for ethylene units present in otherobserved comonomer sequences or end-groups:

E _(total) =E+(5/2)*B

The total mole fraction of 1-butene in the polymer was then calculatedas:

fB=(B _(total)/(E _(total) +B _(total))

The total comonomer incorporation of 1-butene in mole percent wascalculated from the mole fraction in the usual manner:

B [mol %]=100*fB

The total comonomer incorporation of 1-butene in weight percent wascalculated from the mole fraction in the standard manner:

B [wt %]=100*(fB*56.11)/((fB*56.11)+(fH*84.16)+((1−(fB+fH))*28.05))

-   klimke06-   Klimke, K., Parkinson, M., Piel, C., Kaminsky, W., Spiess, H. W.,    Wilhelm, M., Macromol. Chem. Phys. 2006; 207:382.-   pollard04-   Pollard, M., Klimke, K., Graf, R., Spiess, H. W., Wilhelm, M.,    Sperber, O., Piel, C., Kaminsky, W., Macromolecules 2004; 37:813.-   filip05-   Filip, X., Tripon, C., Filip, C., J. Mag. Resn. 2005, 176, 239-   griffin07-   Griffin, J. M., Tripon, C., Samoson, A., Filip, C., and Brown, S.    P., Mag. Res. in Chem. 2007 45, S1, S198-   randall89-   J. Randall, Macromol. Sci., Rev. Macromol. Chem. Phys. 1989, C29,    201.

Rheology

The characterization of polymer melts by dynamic shear measurementscomplies with ISO standards 6721-1 and 6721-10. The measurements wereperformed on an Anton Paar MCR301 stress controlled rotationalrheometer, equipped with a 25 mm parallel plate geometry. Measurementswere undertaken on compression moulded plates, using nitrogen atmosphereand setting a strain within the linear viscoelastic regime. Theoscillatory shear tests were done at T ° C. (T by 230° C. for PP and190° C. for PE) applying a frequency range between 0.0154 and 500 rad/sand setting a gap of 1.2 mm.In a dynamic shear experiment the probe is subjected to a homogeneousdeformation at a sinusoidal varying shear strain or shear stress (strainand stress controlled mode, respectively). On a controlled strainexperiment, the probe is subjected to a sinusoidal strain that can beexpressed by

γ(t)=γ₀ sin(ωt)  (1)

If the applied strain is within the linear viscoelastic regime, theresulting sinusoidal stress response can be given by

σ(t)=σ₀ sin(ωt+δ)  (2)

whereσ₀ and γ₀ are the stress and strain amplitudes, respectivelyω is the angular frequencyδ is the phase shift (loss angle between applied strain and stressresponse)t is the timeDynamic test results are typically expressed by means of severaldifferent rheological functions, namely the shear storage modulus, G′,the shear loss modulus, G″, the complex shear modulus, G*, the complexshear viscosity, η*, the dynamic shear viscosity, η′, the out-of-phasecomponent of the complex shear viscosity, η″, and the loss tangent, tanη, which can be expressed as follows:

$\begin{matrix}{G^{\prime} = {\frac{\sigma_{0}}{\gamma_{0}}\cos \; {\delta \mspace{14mu}\lbrack{Pa}\rbrack}}} & (3) \\{G^{''} = {\frac{\sigma_{0}}{\gamma_{0}}\sin \; {\delta \mspace{14mu}\lbrack{Pa}\rbrack}}} & (4) \\{G^{*} = {G^{\prime} + {{iG}^{''\mspace{14mu}}\lbrack{Pa}\rbrack}}} & (5) \\{\eta^{*} = {\eta^{\prime} - {i\; {\eta^{''}\mspace{14mu}\left\lbrack {{Pa} \cdot s} \right\rbrack}}}} & (6) \\{\eta^{\prime} = {\frac{G^{''}}{\omega}\mspace{14mu}\left\lbrack {{Pa} \cdot s} \right\rbrack}} & (7) \\{\eta^{''} = {\frac{G^{\prime}}{\omega}\mspace{14mu}\left\lbrack {{Pa} \cdot s} \right\rbrack}} & (8)\end{matrix}$

Besides the above mentioned rheological functions one can also determineother rheological parameters such as the so-called elasticity indexEI(x). The elasticity index EI(x) is the value of the storage modulus,G′ determined for a value of the loss modulus, G″ of x kPa and can bedescribed by equation 9.

EI(x)=G′ for (G″=x kPa)[Pa]  (9)

For example, the EI (5 kPa) is the defined by the value of the storagemodulus G′, determined for a value of G″ equal to 5 kPa.The determination of so-called Shear Thinning Indexes is done, asdescribed in equation 10.

$\begin{matrix}{{{SHI}\left( {x/y} \right)} = {\frac{{Eta}^{*}\mspace{14mu} {for}\mspace{14mu} \left( {G^{*} = {x\mspace{14mu} {kPa}}} \right)}{{Eta}^{*}\mspace{14mu} {for}\mspace{14mu} \left( {G^{*} = {y\mspace{14mu} {kPa}}} \right)}\lbrack{Pa}\rbrack}} & (10)\end{matrix}$

For example, the SHI (2.7/210) is defined by the value of the complexviscosity, in Pa·s, determined for a value of G* equal to 2.7 kPa,divided by the value of the complex viscosity, in Pa·s, determined for avalue of G* equal to 210 kPa. The values are determined by means of asingle point interpolation procedure, as defined by Rheoplus software.In situations for which a given G* value is not experimentally reached,the value is determined by means of an extrapolation, using the sameprocedure as before. In both cases (interpolation or extrapolation), theoption from Rheoplus “-Interpolate y-values to x-values from parameter”and the “logarithmic interpolation type” were applied.

REFERENCES

-   [1] Rheological characterization of polyethylene fractions”    Heino, E. L., Lehtinen, A., Tanner J., Seppälä, J., Neste Oy,    Porvoo, Finland, Theor. Appl. Rheol., Proc. Int. Congr. Rheol, 11th    (1992), 1, 360-362-   [2] The influence of molecular structure on some rheological    properties of polyethylene”, Heino, E. L., Borealis Polymers Oy,    Porvoo, Finland, Annual Transactions of the Nordic Rheology Society,    1995-   [3] Definition of terms relating to the non-ultimate mechanical    properties of polymers, Pure & Appl. Chem., Vol. 70, No. 3, pp.    701-754, 1998

Experimental Catalyst Preparation Complex Preparation:

87 kg of toluene was added into the reactor. Then 45.5 kg Bomag A inheptane was also added in the reactor. 161 kg 99.8% 2-ethyl-1-hexanolwas then introduced into the reactor at a flow rate of 24-40 kg/h. Themolar ratio between BOMAG-A and 2-ethyl-1-hexanol was 1:1.83.

Solid Catalyst Component Preparation:

275 kg silica (ES747JR of Crossfield, having average particle size of 20mm) activated at 600° C. in nitrogen was charged into a catalystpreparation reactor. Then, 411 kg 20% EADC (2.0 mmol/g silica) dilutedin 555 litres pentane was added into the reactor at ambient temperatureduring one hour. The temperature was then increased to 35° C. whilestirring the treated silica for one hour. The silica was dried at 50° C.for 8.5 hours. Then 655 kg of the complex prepared as described above (2mmol Mg/g silica) was added at 23° C. during ten minutes. 86 kg pentanewas added into the reactor at 22° C. during ten minutes. The slurry wasstirred for 8 hours at 50° C. Finally, 52 kg TiCl₄ was added during 0.5hours at 45° C. The slurry was stirred at 40° C. for five hours. Thecatalyst was then dried by purging with nitrogen.The polymers of the invention were prepared as outlined in table 1 in aBorstar process (i.e. a loop followed by gas phase process) using thecatalyst above and TEAL cocatalyst:

Table 2 below shows the comparison of inventive examples with somecommercially available prior HDPE grades intended for production ofbeverage closures. It can be concluded that none of competition productshas an optimal balance of rheological and mechanical properties requiredfor the production of beverage closures. The best combination ofproperties is achieved by inventive example no. 1.

Competitor Grades:

-   -   1. ACP5331H (Basell),    -   2. M200056 (Sabic),    -   3. C430A (Samsung)    -   4. CC453 (Sabic)    -   5. Commercial bimodal HDPE

TABLE 2 Comparison of inventive examples with some prior commercialgrades. Mn Mw wt % composition composition Mw/Mn CIS HMW MFR2 MFR5Density (LMW (LMW Mz (LMW SHI (23° C.) fraction g/10 min g/10 min kg/m³fraction) fraction) composition fraction) Mz/Mw 2.7/210 kJ/m² Inv Ex 149 4.3 14.8 957.4 7445 83100 409500 11.2 (5.9) 4.9 13.2 6.5 (4500)(26550) Inv Ex 2 54 4.3 14.3 958 8055 83500 418000 10.4 (6.0) 4.5 11.86.7 (4800) (28800) Inv Ex 3 60 4.4 14.7 957.3 9115 84050 396000 9.2 4.79.7 6.8 M200056 21 57 956.4 12527 47900 156290 3.8 3.26 3.3 3.8 CC4534.3 12 951.3 3.7 C430A 1.8 6.1 957.3 9475 101860 585390 10.8 5.75 17.510.1 ACP5331H 2.1 6.4 955.4 15472 91161 351027 5.9 3.85 6.5 17.5Commercial 4 954 bimodal HDPE flow flow flow flow flow flow lengthlength length length length length Bell 600 bar, 1000 bar, 1400 bar, 600bar, 1000 bar, 1400 bar, ESCR cm cm cm cm cm cm Drawback 190° C. 190° C.190° C. 220° C. 220° C. 220° C. Inv Ex 1 41 12.2 17.7 22.9 13.5 19.324.8 Inv Ex 2 28 11.6 16.9 21.9 13 18.7 23.7 Inv Ex 3 37 11.4 16.4 20.912.7 18.2 23.2 M200056 1.2* 13.7 19 24.5 15.7 21.1 26.4 Low impact, lowstress cracking resistance CC453 8.8 12.2 15.1 Poor flow, poor stiffness(density) C430A 9.8 14.3 19.1 11.3 16.4 21.4 Insufficient flow ACP5331H7.8 11.4 15.4 9.5 13.5 17.5 Poor flow, insufficient stiffness (density)Commercial 11.2 15.8 20.8 12.9 18.2 23.5 Poor flow, insufficient bimodalstiffness (density) HDPE

1. A multimodal polyethylene copolymer comprising: (I) 35 to 55 wt % ofa lower molecular weight ethylene homopolymer component having an MFR₂of 200 to 400 g/10 min; (II) 45 to 65 wt % of a higher molecular weightethylene copolymer component of ethylene and at least one C3-12 alphaolefin comonomer; wherein said multimodal polyethylene copolymer has adensity of 955 to 962 kg/m³, an MFR₂ of 2.0 to 10 g/10 min and an Mw/Mnof more than 9.0.
 2. A multimodal polyethylene copolymer as claimed inclaim 1 having an Mw/Mn of more than 10.0.
 3. A multimodal polyethylenecopolymer as claimed in claim 1 having MFR₂ 2.0 to 4.9 g/10 min,preferably 2.5 to 4.9 g/10 min, more preferably 3.0 to 4.9 g/10 min. 4.A multimodal polyethylene copolymer as claimed in claim 1 prepared usinga Ziegler Natta catalyst.
 5. A multimodal polyethylene copolymer asclaimed in claim 1 wherein said HMW copolymer component comprises atleast one C3-12 alpha olefin, preferably but-1-ene, hex-1-ene andoct-1-ene.
 6. A multimodal polyethylene copolymer as claimed in claim 1having 45 to 60 wt %, preferably 48 to 52 wt % of a HMW component (II)and 40 to 55 wt %, preferably 52 to 48 wt % LMW component (I).
 7. Amultimodal polyethylene copolymer as claimed in claim 1 having a shearthinning index (SHI 2.7/210) in the range of 7.0 to 18.0, preferably 8.0to 15.0.
 8. A multimodal polyethylene copolymer as claimed in claim 1wherein said copolymer is a copolymer with the comonomer 1-butene.
 9. Amultimodal polyethylene copolymer as claimed in claim 1 having an ESCRof more than 20 hrs, such as 20 to 59 hrs.
 10. A multimodal polyethylenecopolymer as claimed in claim 1 having a charpy impact strength measuredat 23° C. of 4.0 to 10.0 kJ/m², such as 5.0 to 8.0 kJ/m².
 11. Amultimodal polyethylene copolymer as claimed in claim 1 wherein thespiral flow length under conditions of 190° C. at 1000 bar pressure is14.0 to 21.0 cm, preferably 16.0 to 19.0 cm and/or the spiral flowlength under conditions of 220° C. at 1000 bars is 16.0 to 24.0 cm,preferably 17.5 to 20.0 cm.
 12. A multimodal polyethylene copolymer asclaimed in claim 1 having a density of 956 to 962 kg/m³, preferably 956to 959 kg/m³.
 13. A multimodal polyethylene copolymer as claimed inclaim 1 comprising: (I) 40 to 55 wt % of a lower molecular weightethylene homopolymer component having an MFR₂ of 200 to 400 g/10 min;(II) 45 to 60 wt % of a higher molecular weight ethylene copolymercomponent of ethylene and at least one C3-12 alpha olefin comonomer;wherein said multimodal polyethylene copolymer has a density of 956 to959 kg/m³, an MFR₂ of 2.0 to 4.9 g/10 min, an ESCR of 20 to 59 hrs, anSHI (2.7/210) of 8.0 to 15.0 and an Mw/Mn of more than 9.0; preferablywherein the spiral flow length under conditions of 190° C. at 1000 barpressure is 16.0 to 21.0 cm; and/or the spiral flow length underconditions of 220° C. at 1000 bars is 17.5 to 20.0 cm.
 14. An injectionor compression moulded article, such as a cap or closure e.g. for a noncarbonated beverage container, comprising a copolymer as claimed inclaim
 1. 15. Use of the copolymer as claimed in claim 1 in themanufacture of a injection or compression moulded article, such as a capor closure e.g. for a non carbonated beverage container.
 16. A processfor the preparation of a polyethylene copolymer as claimed in claim 1comprising; polymerising ethylene in the presence of a Ziegler Nattacatalyst so as to form 35 to 55 wt % of a lower molecular weightethylene homopolymer component having an MFR₂ of 200 to 400 g/10 min;and subsequently polymerising ethylene and at least one C3-12 alphaolefin comonomer in the presence of component (I) and in the presence ofthe same Ziegler Natta catalyst so as to form 65 to 45 wt % of a highermolecular weight component (II).